To measure transparent or partially transparent samples, for example of the human eye, short-coherence interferometers which operate by means of optical coherence tomography (hereinafter: OCT) are known, for example from WO 2007/065670 A1. They serve to detect the location and size of scattering centres within a sample, such as for example miniaturized optical components or biological tissue, e.g. the human eye. For an overview of the corresponding literature on OCT, reference may be made to US 2006/0109477 A1. This patent publication, which partly goes back to one of the inventors of the invention relevant here, also describes the basic principles of OCT.
The principle of OCT comprises both embodiments in which irradiation and radiation detection occur by scanning at different locations across the direction of incidence of the radiation and embodiments, simplified compared with these, in which the irradiation and radiation detection is carried out only along an axis that remains unaltered and axial (i.e. 1-dimensional) scattering profiles are thus generated. The latter embodiment corresponds, as far as the image production is concerned, to a so-called A-scan of ultrasound image production; it is also called optical coherence domain reflectometry (OCDR). When OCT is mentioned here, it is to be understood to mean both scanning and OCDR systems.
Essentially three variants are known for OCT: in time domain OCT, the eye is illuminated by a short-coherent radiation, and a Michelson interferometer ensures that radiation scattered back from the eye can interfere with radiation which has passed through a reference beam path. This principle, already described at a relatively early stage in Huang, et al., Science 254: 1178-1181, 1991, can achieve a depth-resolved image of the sample if the length of the reference beam path is adjusted, whereby a window corresponding to the coherence length of the radiation used is adjusted in the sample. The size of this window defines the maximum achievable depth resolution. For a good depth resolution, radiation sources with the shortest possible coherence, i.e. spectrally wide, are thus necessary. Because of the measurement method, only a fraction of the radiation reflected back, i.e. that scattered back from the measurement depth of the sample, which corresponds to the length of the reference beam path, is detected at any time. In known structures, therefore, over 99% of the photons scattered back from the sample are not actually detected for the measurement.
A higher yield is obtained with another OCT variant, frequency domain OCT. Here, the length of the reference beam path is no longer altered; instead, the radiation brought to interference is detected spectrally resolved. The depth information of the sample, i.e. the depth-resolved contrast signal, is calculated from the spectrally resolved signal. As a mechanism for adjusting the path length of the reference beam path is no longer necessary, the FD-OCT technique is capable of measuring simultaneously at all depths of the sample. The thereby achieved higher yield of the radiation scattered back achieves a sensitivity up to 20 dB higher for the same measurement time. A disadvantage of FD-OCT systems is the maximum measurement range size, which is limited by the spectrometer resolution, and the reduction in sensitivity which increases with the measurement depth. The required structure is also much more expensive.
The SS-OCT variant, in which the spectral resolution of the interference signal with a spectrometer is dispensed with and, instead, the illumination source is spectrally tuned, requires somewhat less additional structural outlay. This method is more sensitive than TD-OCT because of the higher photon yield, as M. Choma et al. explain in “Sensitivity advantage of swept source and Fourier domain optical coherence tomography,” Opt. Express 11, 2183-2189 (2003). In the case of SS-OCT, too, the maximum resolution corresponds to the tunable wavelength range of the radiation source, and the measurement range is predetermined by the coherence length of the radiation used.
In all OCT variants, the measurement range and the measurement resolution are thus linked in a certain way. To rectify the limitation imposed by this, WO 2007/065670 A1 skillfully describes combining several interferometer arrangements which are each assembled from their own reference beam path as well as an associated sample beam path. By different matching of these several interferometer arrangements which, although combined in one apparatus, are independent, measurements can be taken simultaneously at different points in the eye and thus the measurement range can be enlarged. The document further describes different approaches for differentiating the radiations in the combined interferometers, for example in respect of the polarization of the radiation or the wavelength. Such a differentiation is also described in WO 2001/038820 A1 which, however, is concerned only with TD-OCT, thus requires moving elements for adjusting the length of the reference beam path. The principle of using several reference beam paths of different lengths can also be found in US 2005/0140981, or in U.S. Pat. No. 6,198,540, which each relate to TD-OCT for enlarging the measurement range and use several, individually adapted reference beam paths of different lengths.